1. Field of the Invention
The invention is generally related to electrostatic chucks used in high vacuum applications for patterning and processing substrates such as semiconductor chips and wafers. More particularly, the invention is concerned with reducing in-plane distortion of masks when utilizing an electrostatic chuck.
2. Description of the Prior Art
Electrostatic chucking of substrates is a process whereby an electric potential is applied between a substrate and a chuck is used to secure the substrate in place on the chuck during patterning of the substrate, or, in the case where the substrate is a mask, during patterning of a chip or wafer or the like with the chucked mask. The electric potential is maintained during patterning such that the chucked substrate or mask is held in place with a precisely maintained and controlled x-y position. After patterning, the chucked substrate or mask can simply be removed by terminating the flow of electric potential.
An early example of electrostatic chucking of a substrate is shown in U.S. Pat. No. 3,974,382 to Bernacki. In Bernacki, a lithographic mask is attracted to the surface of an X-ray sensitive polymer by depositing the mask on the surface of the polymer, and then passing a current through the mask to a ground block. The electricity causes the mask to be brought into intimate contact with the X-ray sensitive polymer and deform to the slight irregularities of the surface of the X-ray sensitive polymer. X-ray lithography is no longer being done in the manner prescribed in Bernacki. In particular, today's technology requires that the mask not be distorted, and that the mask not be brought into contact with the wafer.
"Single-sided" and "double-sided" electrostatic chucks are discussed in detail in Kendall, IBM Technical Disclosure Bulletin, Vol. 32, No. 5B, October, 1989. FIG. 1 shows a "single-sided" chuck 1 that is fastened to process equipment 2 using hold-down screws 3 or the like. The substrate 5 is held in position on the chuck 1 by applying a voltage from source 6 to the electrode 7 on the chuck 1 thereby generating an electric field through dielectric 8. FIG. 2 shows a "double sided" chuck where two chucks 9 and 10 are placed back-to-back with ground plane 17 between them. The dielectric layer 12 and electrode 13 form chuck 9 which secures the substrate 11 to the chuck 9. The dielectric layer 15 and electrode 16 form the second chuck 10 which secures the entire assembly to the piece of equipment 18 (e.g., X-Y stage) in which it is being used. The two chucks are separated by the ground plane or layer 17 which prevents fields generated by one of the electrodes from protruding into the region of the other electrode. In operation in a high vacuum system, the lower electrode is powered on first to secure the chuck 10 to the process equipment 18. A substrate 11 is placed on the chuck 9 and secured there by powering on the upper electrode. The substrate 11 can then be processed. After processing, the substrate 11 can be exchanged by powering down the upper electrode, and then powering up the upper electrode once a new substrate is added. The lower chuck electrode remains powered-on throughout these operations to prevent any movement of the chuck. When chuck removal is necessary, the lower electrode is powered off to allow release.
U.S. Pat. No. 5,275,683 to Arami et al. discloses a method to for increasing the clamping force of a rigid electrostatic chuck. In Arami et al., an electrostatic chuck sheet comprised of a conductive film is sandwiched between two larger dielectric films and placed on top of the mount body or susceptor. The larger dielectric films are bonded together at their ends which extend beyond the conductive film, and the bonded regions are positioned on a curved portion of the susceptor angled away from the substrate. The arrangement prevents concentration of the electric field on the rim section.
A problem with all prior art electrostatic chuck designs is that they do not accommodate the non-planar character of the surfaces of many of the masks or substrates which are to be clamped. FIG. 3 shows a rigid chuck 20 on which is positioned a slightly bowed X-ray ring 22 which is to be electrostatically clamped. It should be understood that the chuck 20 can be a single-sided or the top portion of a double sided chuck, and that many other substrates besides X-ray ring 22 can be electrostatically clamped, and would have the same problem as set forth below. The clamping face 24 of the X-ray ring 22 does not lay fiat on the rigid chuck 20. Thus, when the electric field is applied, the clamping action, indicated by arrow 26 and generated by the attractive force between the chuck 20 and X-ray ring 22, causes the clamping face 24 to flatten or conform against the rigid chuck 20. This clamping action 26, in turn, causes in-plane distortion of the mask 28 at the pattern area (not shown). For exemplary purposes, if the substrate is an X-ray ring, and the gap 25 between the chuck 20 and ring 22 were on the order of 4-5 microns, the clamping action 26 would produce motion at the perimeter of the mask on the order of 4-5 microns in the mask, and this would produce hundreds of nanometers of in-plane distortion in the pattern area.
U.S. Pat. No. 4,610,020 to La Fiandra discloses an X-ray mask ring which has been specially machined to accommodate a plurality of kinematic mounts for mounting the ring on an alignment cartridge. Mechanical clamping schemes are commonly used in e-beam and x-ray tools; however, the clamping is not electrostatic, and requires special mechanical features on the substrate and/or clamping device.
It would be advantageous to provide an electrostatic clamp, which has the advantages of not requiring special features to be machined into a substrate and not requiring mechanical operations to be performed for removal and replacement of substrates in the clamp, but which accommodates and addresses the in-plane distortion problem which results when the substrate clamping surface is not planar.